Protection of niobium from oxidation



United States Patent PROTECTION OF NIOBIUM FROM OXIDATION Eugene Wainer,Cleveland Heights, Ohio, assignor to Horizons Incorporated, acorporation of New Jersey No Drawing. Application May 21, 1957 SerialNo. 660,491

4 Claims. (Cl. .7 5174) This invention relates to niobium base alloyshaving a high resistance to oxidation at temperatures up to 2500 F. evenafter prolonged or intermittent exposure to oxidizing atmospheres atsuch temperatures and to a method whereby niobium is protected fromdestructive oxidation.

More particularly, it relates to alloys of niobium with at least oneelement from the group of elements which function as oxidation retardersor stabilizers and which consists of beryllium, titanium, aluminum,zirconium, chromium, silicon and vanadium in amounts up to about 4% byWeight and at least one of a second group of elements which functions asa diffusion barrier and which may be present in amounts from aneffective percentage up to about 0.5% and selected from the groupconsisting of calcium, cerium, erbium, lanthanum, neodymium,praseodymium, lead, thorium, or tin.

Niobium and niobium base alloys possess high temperature strengthproperties which make these metals desirable for many end uses.Unfortunately it has been found that these desirable properties areaccompanied by poor oxidation resistance. -When pure niobium or manyniobium base alloys are exposed to oxidation at high temperatures,coatings form on the surface ranging from Nb O to and including Nb OWith continued exposure, or with intensive oxidation, the coatingsformed become thick and flake off, and thereby expose a fresh surface tofurther oxidation. As a result, the article is ultimately weakened tothe point of failure.

One previously proposed means for overcoming this poor oxidationresistance has been to clad the niobium or niobium base alloy articlewith a suitable oxidation resistant alloy. This approach has not beenentirely successful because diffusion between the cladding material andthe niobium base has often produced brittle interfaces which did notwithstand repeated thermal cycling and which in many instancesultimately failed by actual physical separation of the clad from thebase.

Still another disadvantage in the prior efforts to clad niobium andniobium base materials has been inherent in the brittleness of thecladding materials themselves. This has seriously limited the use ofsuch coatings in applications where the article to be protected issubjected to bending stresses.

One object of this invention is to provide a means for protectingniobium and niobium base alloys against oxidation at elevatedtemperatures and particularly against oxidation in environments in whichthe protective member is subject to repeated heating and cooling and tobending stresses.

Another object of the present invention is to provide a technique forselecting alloy compositions which will result in members possessing acombination of inherent properties which prevent the destructiveoxidation of the niobium base material.

In order to develop a system applicable to the protection of niobium andniobium base alloys, it has been found that a double barrier must beprovided to accomplish the required properties. Two distinct functionsmust be performed by the alloying elements added to the base whetherapplied as coating material or incorporated into the base as an alloyingaddition. The first of these functions is to retard or stabilize theoxidation and the second is to bar diffusion. The two additives may bedescribed in terms of their chemical and physical properties relative toniobium and its compounds.

The first additive characterized by me as a stabilizer or retarder mustbe selected from elements having atomic volumes or radii less than theatomic volume or radius of niobium, so that ions or atoms of theadditive will diffuse as fast as the base metal. Furthermore, the heatsof formation of the oxides of the additives should be preferably greaterthan the heat of formation of the niobium oxide which forms when thebase metal is exposed to oxidation.

The second additive, characterized by me as a diffusion barrier, must beselected from elements whose atomic volumes are greater than themolecular volume of the oxide which first appears on the surface of theniobium base article when it is exposed to oxidation. Furthermore, thisadditive should be selected from elements whose oxides have high heatsof formation relative to the heat of formation of niobium oxide andwhose oxides are refractory in nature and are capable of formingcompounds with the first oxide which appears on the surface to beprotected. The resulting compounds must themselves be refractory innature for the proper protection to be achieved.

When an alloy has been prepared with suitable content of an element orelements from the first above described group and the second abovedescribed group, the following process takes place as soon as theresulting alloy is heated in an oxidizing atmosphere. First, an oxidewill appear on the surface of the article and, in niobiumrich systemssuch as those presently contemplated in which the niobium contentcomprises substantially at least by weight of the alloy, this oxide willbe niobium monoxide. On continued heating the atoms of the stabilizer orretarder additive or additives diffuse rapidly through the superficialfilm of niobium oxide to the outside and thereupon form an oxide as aresult of contact with the oxygen or oxidizing environment. In additionto protecting the base metal from further oxidation by the formation ofthis oxide, the additive atoms which are present in the niobium oxidelattice maintain the structure and composition of the niobium monoxideand thus avoid the continued destructive oxidation characteristic ofuntreated base material. In this connection the higher heat of formationof these oxides relative to the heat of formation of the niobium oxideis additionally responsible for maintaining the stability of the niobiummonoxide film.

Suitable stabilizers, as has been previously indicated, may consist ofone or more of the following: beryllium, titanium, aluminum, zirconium,chromium, silicon and vanadium.

Each of the recited elements is capable of reducing higher oxides ofniobium to the monoxide and even to the metal and each of thestabilizers listed may be added from effective amounts up to about 4% byweight of a total of the listed elements.

As a result of diffusion of both niobium and stabilizer atoms from thesurface of the base metal into the oxide layer, a substantial increaseof concentration of atoms of the second additive, above described by meas the diffusion barrier, is available just at the surface on the basemetal under the oxide layer.

The second additive is selected from those elements having atomicvolumes greater than the molecular volume of niobium oxide, and as aresult it blocks any openings that may be available in the niobium oxidestructure. As a result, the niobium atoms of the base metal can nolonger diffuse through the niobium oxide structure and the desiredoxidation resistance .will have been achieved. v

Two suitable methods for applying the above described concepts will nowbe readily apparent. In one, the alloy composition may be preparedbymelting the niobium or niobium base alloy in a suitable furnace under acontrolled atmosphere to avoid oxidation .or contamination of the meltand adding thereto a desired retarder or stabilizer in efiective amountsup to 4% by weight and the desired amount of diffusion barrier elementin amounts up to 0.5 by weight. w

A second method of applying this technique is to coat the-niobium orniobium base article after it has been formed with a mixture of powderin a suitable vehicle compounded so as to contain the proper proportionsof retarder and diffusion barrier additives.

When the first method is employed, may be prepared as follows:

The coarsely granular metals (approximately 20 to 40 mesh) are mixed insuitable proportions and compressed into pellets. These pellets are thenfused twice in an arc furnace operating in an argon atmosphere atapproximately half atmospheric pressure. The arc furnace consistsof awater-cooled copper hearth and a water-cooled tungsten tip. The doublemelting is utilized to insure uniformity. The buttons obtained arerolled into sheet and the edges of the sheet cropped. The analyses givenin the table below are those obtained on the finished a1- loy. Theoxidation resistance is determined qualitatively by examination ofspecimens after a 3-hourexposure in air at 2000 F. and 2500 F.

Suitable alloys for the purpose and an indication of their oxidationresistance are given in the following table.

he desired alloys Wt. Wt. Wt. Oxidation Resistance Percent PercentPercent Retarder Stabilizer Nb the patent statutes, I claim:

1. The method of protecting niobium from destructive oxidation whichcomprises: forming an alloy consisting essentially of between 0.5% and4% by weight of at least one element from the group consisting ofberyllium, titanium, aluminum, zirconium, chromium, silicon and vanadiumand between about 0.1% and 0.5% by weight of at least one element fromthe group consisting of calcium, cerium, erbium, lanthanum, neodymium,praseodymium, lead, thorium, and tin; balance substantially all niobium.

2. The method of protecting niobium and niobium base alloys fromdestructive oxidation which comprises: incorporating at least in theregion of the outer surface of a member formed of said metals between0.5% and 4% by weight of at least-one element retarding the oxidation ofniobium and niobium base alloys and selected from those elements havingan atomic volume less than the molecular volume of niobium monoxide andbetween 0.1% and 0.5 by weight of at least one element serving asbarrier to the diffusion of niobium oxide in said alloys and selectedfrom those elements having an atomic volume greater than the molecularvolume of niobium monoxide.

3. An oxidation resistant niobium base alloy consisting of between about0.5 and 4% by weight of at least one element from the group consistingof beryllium, titanium, aluminum, zirconium, chromium, silicon, andvanadium, and between about 0.1% and 0.5 by weight of at least oneelement from the group consisting of calcium, cerium, erbium, lanthanum,neodymium, praseodymium, lead, thorium and tin, balance essentially allniobium. I

4. An oxidation resistant niobium base alloy consisting of between about0.5 and 4% by weight of aluminum, between about 0.1% and 0.5 of calcium,balance essentially all niobium. I

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES United States Atomic Energy Commission (BMI 1003),publication on Initial Investigation of Niobium and Niobium-Base Alloys,written by Battelle Memorial Institute, Columbus, Ohio, June 23, 1955(pages 28 and 30).

1. THE METHOD OF PROTECTING NIOBIUM FROM DESTRUCTIVE OXIDATION WHICHCOMPRISES: FORMING AN ALLOY CONSISTING ESSENTIALLY OF BETWEEN 0.5% AND4% BY WEIGHT OF AT LEAST ONE ELEMENT FROM THE GROUP CONSISTING OFBERYLLIUM, TITANIUM, ALUMINUM, ZIRCONIUM, CHROMIUM, SILICON AND VANADIUMAND BETWEEN ABOUT 0.1% AND 0.5% BY WEIGHT OF AT LEAST ONE ELEMENT FROMTHE GROUP CONSISTING OF CALCIUM, CERIUM, ERBIUM, LANTHANUM, NEODYMIUM,PRASEODYMIUM, LEAD, THORIUM, AND TIN; BALANCE SUBSTANTIALLY ALL NIOBIUM.